1. Field of the Invention
The present invention relates to a protection circuit, and more particularly, to an electrostatic discharge protection circuit.
2. Background of the Related Art
Generally, an electrostatic discharge protection circuit protects an internal circuit against electrostatic discharge, breakdown voltage, and surge voltage. A related art electrostatic discharge protection circuit will be described with reference to FIG. 1, which is a circuit diagram of the related art electrostatic discharge protection circuit.
Referring to FIG. 1, the related art electrostatic discharge protection circuit includes an internal circuit 11, first and second PN junction diodes 12 and 13 disposed at an input stage of the internal circuit 11, and third and fourth PN junction diodes 14 and 15 disposed at an output stage of the internal circuit 11. An anode of the first PN junction diode 12 is connected to the input stage of the internal circuit 11 and a cathode thereof is connected to a power source voltage stage. An anode of the second PN junction diode 13 is connected to the input stage of the internal circuit 11 and a cathode thereof is connected to a power source voltage stage. An anode of the third PN junction diode 14 is connected to the ground stage and a cathode thereof is connected to an output stage of the internal circuit 11. An anode of the fourth PN junction diode 15 is connected to the output stage of the internal circuit 11 and a cathode thereof is connected to the power source voltage stage.
The first and second PN junction diodes 12 and 13 are protection diodes that protect the internal circuit 11 against the electrostatic discharge applied to the input stage of the internal circuit 11. The third and fourth PN junction diodes 14 and 15 are protection diodes that protect the internal circuit 11 against the electrostatic discharge applied to the output stage of the internal circuit 11.
The operation of the related art electrostatic discharge protection circuit will now be described. As illustrated in FIG. 1, if a negative (-) electrostatic discharge is applied to the input stage of the internal circuit 11, the first PN junction diode 12 is turned on. As a result, the negative electrostatic discharge is bypassed to the ground stage through the first PN junction diode 12. If a positive (+) electrostatic discharge is applied to the input stage of the internal circuit 11, the second PN junction diode 13 is turned on. As a result, the positive electrostatic discharge is bypassed to the power source voltage stage through the second PN junction diode 13.
Meanwhile, if the negative (-) electrostatic discharge is applied to the output stage of the internal circuit 11, the third PN junction diode 14 is turned on. As a result, the negative electrostatic discharge is bypassed to the ground stage through the third PN junction diode 14. If the positive (+) electrostatic discharge is applied to the output stage of the internal circuit 11, the fourth PN junction diode 15 is turned on. As a result, the positive electrostatic discharge is bypassed to the power source voltage stage through the fourth PN junction diode 15.
Thus, the related art electrostatic discharge protection circuit has two PN junction diodes respectively at the input and output stages of the internal circuit 11 to protect the internal circuit 11 against the negative or the positive electrostatic discharge.
However, the related art electrostatic discharge protection circuit has various disadvantages. Since a discharge path that protects the internal circuit depends on a junction area of the PN junction diode and its density, the junction area of the PN junction diode must be greater in the case that a strong electrostatic discharge is applied. In addition, if junction breakdown of the PN junction diode occurs because of a strong electrostatic discharge, it is difficult to realize the discharge path. As a result, it is not possible to protect the internal circuit.